DAAs are used to provide a dielectrically isolated interface between a telephone line pair (tip and ring pair) and a telephone device such as a modem or FAX machine. A DAA is typically divided into a line side circuit for interfacing to the tip and ring contacts and a modem side circuit for interfacing to the modem or computing equipment. FIG. 1 is a functional block diagram illustrating an example of a DAA 1 that includes a line side circuit 10 for interfacing with TIP and RING terminals and a modem side 30 for interfacing to data transmit TX and receive RX terminals. There are several functions which a DAA is generally required to perform.
Typically, the line side circuit of the DAA should present a controlled AC (alternating current) impedance of approximately 600 ohms. Another requirement is that the DAA draw a holding current in the range of 20 to 120 milliAmperes (mA) to signal an off-hook state at the tip and ring. In some countries, regulatory requirements are that the DAA needs to limit the holding current to 50–60 mA. By drawing the holding current from the telephone line, the DAA signals the central telephone office that it is active (off-hook) to either originate or answer a communications connection with the central office. A DAA also typically provides for ring detection and sometimes auxiliary line status functions; such as, line in use, loop current detection, line reversal detection, on-hook audio monitoring for Caller ID functions, etc. A DAA may also provide a 2–4 wire hybrid function. Finally, a DAA must provide all these functions across a high voltage dielectric isolation barrier.
In the on-hook state, the line side circuit typically may draw no more than a few milliamperes of current in order to avoid erroneously signalling the central office. In addition, for line maintenance, to facilitate identifying line leakage, telephone devices are required by regulation (the exact requirements depending on the country) to restrict on-hook DC resistance to typically over 1 Megaohms (MΩ), which limits the current available for on-hook functions to less than a few microamperes. For example, in the US, FCC part 68 limits on-hook DC resistance to over 5MΩ.
When off-hook, DAAs must provide a correct AC terminating impedance to the telephone line pair to allow proper central office two-to-four wire hybrid balance in order to minimize echoing. This leads to an impedance value, as noted above, of around 600 ohms in the United States and most of the world.
Another requirement of DAAs is to provide dielectric isolation between the telephone line voltages and the local ground potential because the telephone line pair power supply is usually geographically separated from the DAA and significant ground potential differences may arise. Consequently the telephone line pair ground potential is unterminated at the DAA. The DAA therefore isolates the line side circuit from the modem side circuit.
Also, the line side circuit must deal with a different scale of voltages from the modem side circuit. For example, the loop voltage on the line pair in the United States is typically −48 Volts Direct Current (VDC). The ringing voltage in the US is typically a 20 Hertz (Hz) 88 Volt RMS signal. Also, the line side circuit may be subject to overvoltage conditions that may arise due to lightning strike or a power line cross.
The modem side circuit, typically, operates at a supply voltage of, for example, 3 to 5 VDC and includes components that are subject to damage if exposed to the voltages present in the line side circuit. The DAA typically includes a two to four wire hybrid that provides first order transmit echo signal cancellation.
Consequently, the line side and modem side circuits are typically isolated from one another. One conventional approach to achieving isolation is to use a transformer, such as transformer 20 illustrated in FIG. 1, to separate the line side and modem side circuits. U.S. Pat. No. 5,369,666 to Folwell et al. for a Modem with Digital Isolation and U.S. Pat. No. 5,790,656 to Rahamin for a Data Access Arrangement with Telephone Interface, herein incorporated by reference in their entirety for all purposes, are two examples of approaches using transformers for isolation. Opto-isolators and capacitors are also used to provide an isolation barrier. U.S. Pat. No. 5,946,393 to Holcombe, herein incorporated by reference in their entirety for all purposes, shows an example of a DAA that can function with an opto-isolator. U.S. Pat. No. 5,500,895 to Yurgelites, herein incorporated by reference in their entirety for all purposes, shows an example of a DAA configured to function with capacitors that provide isolation. FIG. 2 illustrates an example of a DAA 40 that utilizes capacitors for isolation. A line side circuit of DAA 40 is AC signal coupled to a modem side circuit 44 via isolation capacitors 45, 48, 50 and 52.
U.S. Pat. No. 5,654,984 to Hershbarger et al. and U.S. Pat. Nos. 5,870,046; 6,107,948; and 6,137,827 to Scott et al., herein incorporated by reference in their entirety for all purposes, show further examples of DAA isolation techniques configured to function with capacitors that provide isolation wherein high-speed digital signals are used to transmit data across the isolation barrier. A DAA must transmit signals between the attached modem device and the telephone line pair. Transmitting signals across the isolation barrier is a significant challenge in the design of DAAs. The isolation barrier prevents direct coupling of signals between the line side and the modem side circuits. Also, the signals passed across the isolation barrier are subject to strong common mode noise signals.
For those skilled in the art, a wide variety of isolation techniques have been explored including, for example, bidirectional transformers, unidirectional transformers, modulated carriers with use of transformers, baseband bi-directional transformers, baseband bi-directional capacitors, or audio modulated sub-carriers. In some applications, analog signals are sent across the isolation barriers. In other applications digitized signals that represent discrete bits are transmitted across the barrier (as disclosed in the patents referenced above). Because analog baseband audio signals are in the low frequency range of 200 Hz–4 kHz, these frequency signals require large coupling devices for isolation in order to allow transmission. Small scale coupling devices may be used with higher frequency signals, but this requires modulating or encoding the baseband signal onto a high frequency carrier signal suitable for transmission across the isolation barrier. One method is to digitize the audio with a high data rate delta sigma bit stream and send these bits across the isolation barrier. See Hershbarger et al. and the patents of Scott et al. for examples of signals that are delta sigma converted for transmission. It is generally understood in the art that capacitive coupling of an encoded or carrier modulated signal is the lowest cost isolation solution because the capacitors required are of lower cost and smaller than other alternative coupling techniques.
However, discrete high voltage capacitors are relatively expensive devices, especially if they need to be highly matched, and, in addition, discrete component require assembly on a printed circuit board. Larger and more expensive than standard high voltage capacitors are those designed to meet standards of European Norm EN60950 or similar electrical safety standards since they must either (1) meet structural minimum insulation thickness requirements of 0.4 mm and minimum creepage distance of 2.5 millimeters or (2) if standard capacitors are used, multiple caps must be used and placed in series, thereby increasing cost and board area.
Although small capacitors, such as those which can be implemented on a circuit board are known to work, use of such capacitors requires highly specialized line powered design techniques to allow power up and line side on-hook operation without drawing excessive current from the telephone line pair. Large capacitors, over 300 pF, are used in some approaches to provide higher current levels to directly power both hook control and circuitry active when on-hook, but these larger devices incur greater cost and size limitations. In addition, due to the larger current levels of the signals sent across the capacitors, highly balanced differential drivers and highly balanced capacitors need to be used to prevent common mode noise exceeding regulatory limits.
When off-hook, one conventional approach to transmitting audio signals across the isolation barrier with good noise immunity involves encoding signals using pulse width modulation (PWM) encoding and transmitting the resulting edges across the isolation barrier using a differential amplifier. FIG. 3 is a functional block diagram illustrating the approach of U.S. Pat. No. 4,835,486 to Somerville, herein incorporated by reference in its entirety for all purposes, which shows an example of an isolation amplifier 60 used to transmit signals across a capacitive isolation barrier. Isolation amplifier 60 includes a PWM encoder 62 that encodes a digital data input signal to produce a PWM encoded digital signal that is input to differential driver 64. The resulting PWM encoded digital signal tends to have intermediate frequency content to the PWM encoded edges. These edges are differentially amplified by driver 64 that drives the resulting differential digital PWM signal across isolation capacitors 46 and 48, where they are received by differential receive amplifier 70. Differential receive amplifier 70 converts the differential digital PWM signal to a single ended received digital PWM signal that is input to comparators 72 and 74. Comparator 72 compares the single ended received digital PWM signal to a positive threshold voltage VTH+ to produce a logical high signal input to a set terminal of SR flip-flop 76. Comparator 74 compares the single ended received digital PWM signal to a negative threshold voltage VTH− to produce a logical low signal input to a reset terminal of SR flip-flop 76. SR flip-flop 76 converts the output of comparators 72 and 74 into a received digital PWM signal at inverting and non-inverting outputs of flip-flop 76 that, in turn, drives a PWM decoder 78 that decodes the received digital PWM signal into a received digital signal.
The differential approach illustrated in FIG. 3, which is also typical of many prior art designs including U.S. Pat. No. 5,500,895 to Yurgelites, has several drawbacks. One obvious drawback is the requirement for two isolation capacitors to handle both sides of the differential signal transmitted from driver 64 to receive amplifier 70. The market for DAAs is highly competitive and the cost required for multiple isolation capacitors per signal path can undermine the competitiveness of the resulting design. Further, Yurgelites employs an amplitude modulation (AM) system that AM encodes an analog signal received at the telephone line pair. However, AM encoding has generally poor noise immunity and is vulnerable to aliasing from radio frequency signals, most notably from AM radio broadcast stations in the 550 kHz to 1650 kHz band. Low level radio frequency signals also produce audio heterodynes. Since high speed modems need signal to noise ratios of up to 80 dB, even very low level heterodynes can impair performance. In fact, it is this problem which requires that there be a very high level of balance on the capacitors, differential drivers, differential receivers, and board layout to minimize the effects of radio frequency interference.
The solution shown in Scott et al. teaches use of a line side powered digital coder/decoder (CODEC). Although a high speed modem DAA system typically requires a CODEC somewhere in the system, placing the CODEC on the line side adds design difficulties since the line side circuit needs both digital and complex analog circuits that must be powered from the telephone line pair. Also, these circuits all interact with one another and require great design care and specialized techniques to prevent unwanted interactions between the circuits and to prevent circuit load noise from getting back onto the telephone line pair. Holcombe's DAA patent teaches some of these techniques.
Another issue with solid state DAAs intended for use with high speed modems is that due to the fact that the line side is typically powered from the telephone line pair and is analog in nature, this circuit is more difficult to design and often requires more revisions than the modem side IC. Also, because of minimum operating voltage requirements and complex analog requirements, it is desirable to produce the line side circuit using a bipolar complementary metal oxide semiconductor (BICMOS) fabrication process with at least a 3V capability. This is because BICMOS processes provide the best mix of analog components; these being, high density capacitors, high density resistors with a wide resistance ranges, complementary bipolar transistors, and complementary MOS transistors. Use of a fabrication process with a minimum operating voltage of 3–5V is desirable since analog designs intended for operation at or below 2.4V require specialized low voltage design techniques which are not area efficient and are also performance limited in noise, dynamic range, power, bandwidth, etc. A BICMOS process which is cost or performance optimal for an analog design is often not cost or performance optimal for high density digital ICs. Commonly circuits fabricated using digital CMOS processes have lower maximum operating voltages, down to as little as 1.8V, and do not have the rich assortment of analog components available on BICMOS processes.
The present invention is directed toward an improved approach for transmitting signals across an isolation barrier.